[Fix #7712] Fix an incorrect autocorrect for Style/OrAssignment

[koic]

Fixes #7712.

This PR fixes an incorrect autocorrect for Style/OrAssignment when using elsif branch.

The following is a reproduction procedure.

% cat example.rb
foo = if foo
        foo
      elsif
        bar
      else
        'default'
      end

It is changed to the code with syntax error as follows.

% bundle exec rubocop -a --only Style/OrAssignment
(snip)

% cat example.rb
foo ||= elsif
          bar
        else
         'default'

% ruby -c example.rb
example.rb:1: syntax error, unexpected `elsif'
foo ||= elsif

The or assignment operator replaced when using elsif branch is complicated.
The PR will accept the above case without offense.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks!